This invention relates to the flash hydropyrolysis of high ranking caking (agglomerating) coals, such as bituminous coal, to produce gaseous, liquid and solid decomposition products. More particularly, it relates to a process in which high rank caking coals can be flash hydropyrolyzed in a tubular reactor or a fluidized bed reactor.
Pyrolysis or carbonization of coal and other carbonaceous solids is a well-established technique. It comprises heating carbonaceous material to temperatures at which thermal decomposition occurs with the formation of condensible organic liquids, non-condensible gases and solid residue. The condensible organic liquids obtained are normally referred to as tars and light oils, while the solid residue is normally referred to as char. The tar as produced by the process can be further refined with hydrogen to produce a range of liquid fuel products.
The total yields of tar and liquid hydrocarbons from pyrolysis of coal and other carbonaceous material are markedly influenced by pyrolysis conditions such as heating rate, temperature and residence time of the liberated volatiles and coal particles in the pyrolysis zone. When coal is subjected to rapid or flash pyrolysis followed by rapid quenching of the volatile products, the yields of liquids from the process are maximized and secondary decomposition of the tar product is minimized. This concept of flash pyrolysis has been widely accepted as a carbonization technique for the production of oil from coal.
Flash hydropyrolysis must be carried out at very high heating rates of the coal particles and also with very low residence time of the volatiles in the pyrolysis zone. These conditions are readily met by processing finely divided coal particles in either a fluidized bed or entrained flow reactor.
Problems are experienced when caking coals, e.g. bituminous coals, are used in flash pyrolyzers because it is necessary to take the coal particles through a temperature range at which they become plastic, and in which stage the coal particles tend to agglomerate or cake, before good yields of volatiles are obtained. With caking coals, severe build up of caked or agglomerated char can occur in the pyrolyzer or the product outlet lines, or in both. These caked or agglomerated char deposits can adversely affect the operating characteristics of the pyrolyzer and can ultimately render the process inoperable.
Various techniques have been proposed for overcoming or reducing the problems experienced with agglomerating or caking coals. For instance, the caking carbonaceous material may be mixed with non-agglomerating materials such as hot char. One such process is described in Sass et al., U.S. Pat. No. 3,736,233. However, when the caking coal particles are diluted by mixing them with non-agglomerating solid material, such as char, the quantity of recycled char required is excessively large when this material is derived from an external source, and internal recycling of char or other inert material introduces an additional hot surface which enhances the cracking reactions with a resultant loss in char yield. Moreover, the use of a large amount of inert materialreduces the reactor efficiency by occupying a significant portion of the effective reactor volume.
It is an object of the present invention to provide a new and simplified technique for overcoming the agglomeration or caking problems associated with the flash pyrolysis of caking coals.